This invention relates to an electrical power supply circuit for converting alternating current power into a direct current output and of the type including a transformer for electrically isolating the input from the output and for stepping up or down the input voltage to the level required to provide the DC voltage desired at the output terminals, and deals more particularly with an improved power supply circuit of the foregoing type which further includes a phase control switching means in the primary winding circuit for controlling the conduction angle during those half cycles of input voltage in which conduction takes place through the primary windings.
The general type of power supply circuit with which this invention is concerned is shown by prior U.S. Pat. No. 3,466,527. It includes a transformer having a primary winding adapted for connection to a source of alternating current power, a rectifier connected to the secondary winding of the transformer, a filter capacitor connected across the output of the rectifier and in parallel with the load, a phase control switching device in the primary circuit, and a firing angle control circuit responsive to the output or load voltage for controlling the firing angle of the switching device. The invention may be applied to a circuit wherein the rectifier is a half wave rectifier and the phase control switching device a unidirectional device, so that conduction through the transformer primary winding is limited to alternate half cycles of the input voltage. Preferably, however, and as shown herein, the circuit is one utilizing a full wave rectifier and a bidirectional phase control switching device so that conduction through the primary winding occurs during each half cycle of input voltage.
In the circuit of the aforesaid patent, the full voltage of the AC source appears across the transformer primary winding during substantially all of the time that the phase control switching device is conducting, and during normal operation the firing angle is limited to the last 90.degree. of each half cycle of the input voltage. As an overload current protection feature, the transformer core is sized so that it will not saturate for firing angles occurring in the normal last half of each half cycle range but will saturate for earlier occurring firing angles, thereby producing, when saturation occurs, a surge of input current which positively blows or trips a fuze or other current responsive circuit interrupting safety device. A saturable inductor may be included in the primary circuit, but this inductor saturates soon after the initiation of conduction and has no affect on the primary winding current and voltage throughout most of the conduction angle.
In the power supply circuit of this invention, the inductor in the primary winding circuit is one which does not saturate regardless of the firing angle of the phase control switching device. The inductor, therefore, provides overload protection by limiting the input current to safe levels and by suppressing rapid current changes occurring at any time throughout the conduction angle. Preferably, the circuit components are of such values that the firing or switching ON of the phase control switching device occurs in the last half of each half cycle of the input voltage so that the transformer core need only support a maximum volt-time integral equal to half of the volt-time integral of the input voltage, and the transformer core is preferably of the minimum size necessary to support such maximum volt-time integral. If false triggering of the phase control switching device does occur at a small firing angle and the transformer does saturate, the inductor will limit the current in the primary circuit to a safe level.
Further, since variations in the maximum value of the AC input voltage cause changes in the conduction angle, the transformer primary winding does not see such voltage variations and thus the transformer core need not be oversized to accommodate them. Also, due to the fact that the inductor spreads out the conduction time in each half cycle to lower peak currents, the current ratings of the triac, or other component or components used as the phase control switching means, may be reduced, and wires of smaller size and RMS current rating may be used for the transformer windings, thus further reducing the transformer size and cost. The current through the primary winding is not continuous but instead consists of pulses which start and stop at zero, and the switchings of the phase control device ON and OFF occur at zero current values so that RFI normally associated with such switching devices is greatly reduced.
In the basic circuit of this invention, the conduction angle, during each half cycle, consists of a first portion during which power is transferred from the primary winding to the secondary winding to charge the filter capacitor, and a second portion, occurring after charging of the capacitor ceases, during which no power is transferred between the transformer windings and during which only a small exciting current flows through the primary winding. Although no useful transfer of power is accomplished during the second or primary winding excitation portion of the conduction angle, the primary winding is nevertheless exposed to the input voltage during this time and contributes to the volt-time integral to be supported by the transformer core. Therefore, the basic circuit of this invention may be modified to include a commutator circuit for turning OFF the phase control switching device before the end of the voltage half cycle to avoid exciting the primary winding during all or part of the time of no useful power transfer and to thereby further reduce the volt-time integral and to allow the transformer core size to be correspondingly reduced.
Other objects and advantages of the invention will be apparent from the drawings and from the following description of the preferred embodiments thereof.